The invention is directed to a phased array of optical waveguides for wavelength division multiplexing and demultiplexing of optical signals. In particular, the novel optical waveguide multiplexer/demultiplexer includes an array of waveguides which are configured to efficiently collect light from the phased array, at a spatial location relatively close to the phased array, while minimizing coupling among the waveguides of the output array.
Wavelength division multiplexing and demultiplexing (WDMD) have gained in importance as system data rate and system capacity requirements have increased. Usually, the two types of signal processing, i.e., multiplexing and demultiplexing are discussed together because many embodiments of WDMD devices are reciprocal. That is, multiplexed signals which are transmitted through a WDMD device in a first direction are separated into component wavelengths, each propagating in a pre-selected one waveguide. Alternatively, signals of different wavelengths propagating in separate waveguides are multiplexed when transmitted through the device in a direction opposite the first direction. This reciprocity of function is found in the devices disclosed and described herein. Thus it will be understood throughout this document that a discussion of the multiplexing function includes the reciprocal demultiplexing function. Choosing one or the other function for discussion removes the need for discussion of its obvious reciprocal, thus simplifying the description without sacrificing completeness.
The parts or segments of a WDMD device typically include a phase shifting array of waveguides which can sort signals by wavelength, one or more waveguides for transmitting or receiving multiplexed signals, and a waveguide array for transmitting or receiving a plurality of demultiplexed signals. The transmitting or receiving segments must be properly coupled to the phase shifting array. In particular, the one or more waveguides which launch multiplexed signals into the phase shifting array must be so coupled to the phase shifting array as to uniformly distribute signal intensity onto the ends of the waveguides of the array. It is understood that the intensity of the light in a plane does have some variation. In this context, the term uniform means that the shape the intensity distribution of light may have is not such that unacceptable crosstalk appears in the multiplexed or demultiplexed signals. In addition, proper demultiplexing of signals occurs when light intensity from each of the waveguides of the phase shifting array is distributed uniformly onto each waveguide of the array which propagates a demultiplexed signal. The multiplexing and demultiplexing functions may be described using a different conceptual model. For example, one may speak of the wavefront exiting the phase shifting array as being tilted at an angle which provides for constructive interference of the phase shifted light. A different tilt angle corresponds to different light wavelengths. The results, i.e., the structure of a WDMD is, of course, not effected by the choice of descriptive model. The alternative description is mentioned here to avoid confusion between this application and other publications or previously filed applications.
The characteristics of a satisfactory WDMD device are:
minimum signal attenuation within the device; PA1 separation of signals by wavelength sufficient to meet a pre-selected bit error rate ceiling; PA1 small size; and, PA1 ease of manufacture which can translate into lower cost. A particular one of these characteristics may gain in importance depending upon the requirements of the system in which the device is used. PA1 the at least one input waveguide must distribute the multiplexed signal uniformly among the first ends of the N waveguide phase shifting array; and, PA1 each of the M waveguides of the output array must receive phase shifted signal light from each of the N waveguides of the phase shifting array. This spacing can be reduced by tapering the input or output waveguide end segments or by tapering the first or second ends of the phase shifting array. The reduction or decrease in spacing is in comparison to a WDMD device which has no tapered portion but is of the same general configuration as the novel WDMD. PA1 coupling between the input and the first ends of the phase shifting array must distribute light intensity from the input uniformly to the first ends; and, PA1 coupling between the output ends and the second ends of the phase shifting array must allow light from each second end to interfere at the output ends, thereby effectively delivering to each output end a single signal wavelength. These conditions on the coupling are stated for the demultiplexing function of the device. The reciprocal conditions, which must exist for the multiplexing function are: PA1 coupling between output array ends and phase shifter second ends is such that light from each output array end is delivered substantially uniformly to the second ends; and, PA1 coupling between the phase shifter first ends and the at least one input fiber is such that essentially all the light emerging from the first ends is focussed upon the at least one input waveguide end. PA1 fabricating the phase shifting array; PA1 providing coupling, as described above, between the input and output waveguides and the respective first and second ends of the phase shifting array, wherein the first or second ends of the phase shifting array, the end of the at least one input waveguide or the ends of the output waveguide array are tapered. The direction of larger to smaller taper is always directed toward an optical coupling. The taper can be formed by any of several methods known in the art including etching, ion exchange, or heating and precision stretching of the end portion to be tapered.
There is a strong need within the telecommunications industry for WDMD designs which provide adequate signal separation and low signal attenuation in a device which is relatively small in size and simple to manufacture. U.S. patent application Ser. No. 08/586,134, filed Jan. 11, 1996, discloses a WDMD device which simplifies the spatial configuration of the phase shifting array. The phase shifting function is performed by an array of waveguides each of which has a unique propagation constant, thus eliminating the need for a process in which the waveguides are arranged in a pattern which allows the waveguides to have unique lengths. Because adjacent waveguides of the phase shifting array have different propagation constants, cross talk among the waveguides of the array is small and thus a source of error is removed from the WDMD. Stated differently, because neighboring waveguides have different propagation constants, there is no need to adjust the geometry of the waveguides at their input or output ends to avoid cross talk error. In particular, the spacing between neighboring waveguides in the phase shifting array may be brought closer together, reducing the dimensions of the WDMD without effecting performance.
The novel work disclosed herein is an additional step toward decreasing WDMD component size without negatively effecting either waveguide cross talk or signal attenuation in the device.